Abstract
A vesuvianite sample from Crestmore, California, exhibits violations of space group P4/nnc in selected-area electron diffraction (SAED) pattems. Within the errors of SAED, the patterns are consistent with space group P4/n. The violating diffraction spots are diffuse with a sharp center and are flattened in the [001] direction, suggesting fine-scale domains. Dark-field and high-resolution electron images reveal a pervasive domain structure that is interpreted to form by transformation twinning resulting from ordering of a disordered, high-temperature structure with space group P4/nnc. The domains in most regions are approximately 10-50 nm wide and are elongated parallel to [001], but larger domains in some areas exceed 1 μm in size. Rarely, the domain structure assumes a lamellar morphology, with the lamellae parallel to {100}. Convergent-beam electron diffraction patterns from adjacent domains and reversals in dark-field domain contrast with minor changes in orientation suggest that the true symmetry of this vesuvianite is P2/n or lower. The observation of a fine-scale domain structure in low-symmetry vesuvianite may help to explain difficulties encountered in some X-ray diffraction and TEM studies of these materials.
